When surveyed, almost all Americans say they believe in God, a majority say they pray and more than a third say they go to religious services every week.

But in a new book on religious trends in the United States, a Duke University professor says this picture of an unflinchingly faithful America is not quite accurate. At least when it comes to traditional religious practices, Americans' belief has faded in recent decades, says professor Mark Chaves.

Take the well-known fact that fewer Americans are joining the clergy. The Roman Catholic Church in the United States, for example, has experienced a sharp decline in priests.

Part of the reason, Chaves says, could be that Americans have lost respect for religious leaders. That's one of several findings in his book "American Religion: Contemporary Trends," which is being released Sunday.

Chaves, a professor of sociology, religion and divinity, found that between 1973 and 2008, the percentage of people with "great confidence" in religious leaders declined from 35 percent to less than 25 percent. He also found that two-thirds of Americans say they would prefer religious leaders to stay out of politics.

Using data from the General Social Survey and the National Congregations Study, Chaves looked at developments in American religion since 1972. The General Social Survey, which began that year, is an ongoing look at American attitudes and behaviors by the University of Chicago's National Opinion Research Center, while the National Congregations Study -- a project Chaves directed -- examined congregations in the United States from several religions, including Christianity, Judaism and Islam.

"The American public has lost confidence in leaders of all sorts,” Chaves says. “But the loss of confidence in religious leaders has been more precipitous.”

Americans also have less interest in traditional religious practices than they once did, he adds. The percentage of Americans who attend church or other worship services has declined slightly over the past four decades, Chaves says.

Perhaps more striking is the evidence that many Americans exaggerate their church attendance. About 35 to 40 percent claim to attend church, but only 25 percent actually do. That's based on comparisons between the General Social Survey and the U.S. Bureau of Labor Statistics' American Time Use Survey, which ask, respectively, what Americans generally spend their time doing and what Americans actually spent their time doing on a specific day.

Chaves cites several reasons for this loss of faith in traditional religion, including the ongoing sex-abuse scandal in the Roman Catholic Church, which first broke out in Boston in the early 2000s.

Here are some other findings by Chaves, who looked at trends in religious diversity, belief, involvement, congregational life, leadership and Protestant decline.

In the 1950s, 99 percent of Americans said they believed in God. In 2008, only 92 percent did.

In 1991, 30 percent of Americans "strongly agreed" that religious leaders should not take part in politics. By 2008, that number had jumped to 44 percent.

The extent of an American's religious involvement more closely predicts that person's political leanings today than it did in the past.

“Several decades ago there was not a strong correlation between how religiously active you were and whether you voted Republican or Democrat,” Chaves says. “Now, there is. If you’re religiously active, you’re now more likely to vote Republican.”